This study of deoxycoformycin therapy in patients with hairy cell leukemia or T-gamma lymphoproliferative disorder was written as a salvage protocol to treat these patients if they failed to respond to low-dose alfa interferon on other ongoing CRB protocols. Deoxycoformycin is known to have significant activity in hairy cell leukemia and has been reported to produce high complete response rates in previously untreated patients and to lead to responses in patients failing prior interferon therapy. It has unproven efficacy in the T-gamma lymphoproliferative disorder. Seven patients have been admitted to this study, four with hairy cell leukemia and three with T-gamma lymphoproliferative disorder. All four patients with hairy cell leukemia were refractory to interferon alfa-2a therapy. Three of these four patients had a substantial partial response to dCF therapy and have at this moment only minimal residual hairy cell infiltrate in the bone marrow. They have normalized their peripheral blood counts, and all have been off of deoxycoformycin therapy for a minimum of six months; all remain in stable partial remission off all therapy. One patient with hairy cell leukemia died after the second dose of dCF because of profound thrombocytopenia and refractoriness to HLA- matched platelet transfusions and extensive GI bleeding. At autopsy there had been a substantial reduction in the number of hairy cells in this patient's bone marrow. Of three patients with T-gamma lymphoproliferative disorder, one patient was a complete responder, and two patients were nonresponders. The one complete responder was an elderly woman with severe granulocytopenia. We conclude that deoxycoformycin has activity in interferon-resistant hairy cell leukemia and in some patients with T-gamma lymphoproliferative disorder. Treatment is.associated with acceptable toxicity but can worsen the pancytopenia associated with either of these diseases.